The level of CO2 measured at Mauna Loa passed the 400 ppm level a couple of years ago, May 2013 (Arctic measurements first passed 400 ppm in 2012). At that time, it was for only a short time before the level dropped again as carbon dioxide was absorbed by plant life during the summer months. Last year, it passed the 400 ppm level again, but this time it was for about three months. This year, it passed the 400 ppm level in January and will probably stay there until August. I speculate 2016 may be the last year we ever see it drop below 400 ppm.
However, this was only for the northern hemisphere. The southern hemisphere has lower levels of CO2 so it has remained under the 400 ppm level - until now. NOAA has announced the global CO2 level for March was 400.82 ppm, the first time the entire planet was over 400 ppm. Of note, it has not been that high for over 2 million years. Not only that, it is rising 100 times as fast as natural rises in the past. Before the industrial age, it took 6000 years for the level to increase by 80 ppm. We have increased it by 61 ppm in just 35 years.